The objective of this research project is to establish the extent to which ultrastructural and specific physiological effects of vasopressin are correlated. Freeze-fracture electron microscopy of the toad urinary bladder demonstrates that vasopressin exposure results in a clustering of intramembranous particles. This structural effect is restricted to the luminal membrane of the granular cells which is known to be the site of vasopressin-induced permeability changes. In order to assess the relationship between clustering and the physiological effects of vasopressin the following specific studies are proposed: 1. Characterization and quantitation of the membrane structural effects of vasopressin; 2. Assessment of the function of clustered particles by deep etching; 3. Correlation of clustering with the degree of vasopressin stimulation; 4. Correlation of clustering with urea transport; 5. Correlation of clustering with water permeability.